Helicopters have been heretofore designed with counter-rotating coaxial rotors or propeller assemblies and some of these have included propeller or rotor assemblies of different diameters. Also, counter-rotating propellers have been heretofore provided on conventional propeller driven aircraft and directional controlling assemblies for helicopters have been provided whereby the plane in which the rotor of the helicopter swings may be canted in all directions relative to the vertical. Examples of these prior structures may be found in U.S. Pat. Nos. 1,381,939, 1,879,142, 2,037,745, 2,041,789, 2,185,545 and 3,554,467.